1. Field of the Invention
The present invention provides a method for producing a silicon wafer whereby it is possible to fabricate a wafer both of whose surfaces exhibit a high precision flatness and minute surface roughness, and which allows one to visually discriminate between the front and rear surfaces of the wafer.
2. Description of the Related Art
The conventional method for producing a semiconductor silicon wafer comprises the steps of separating a silicon block from a silicon single-crystal ingot pulled upward, slicing the silicon block to produce wafers, subjecting the wafer to chamfering, mechanical polishing (lapping), etching, and mirror-polishing (polishing), and cleaning the wafer. Thus, a wafer is obtained that has a high precision flatness. The steps constituting the conventional method may be modified in various manners depending on given purposes: some foregoing steps may be exchanged in order for other later steps; a certain step may be repeated two or more times; a heating step or a scraping step may be added or put in the place of an existent step.
A wafer having undergone mechanical processing such as block separation, peripheral abrasion, slicing, lapping, etc., suffers from processing deformations on its surfaces, or has degraded superficial layers which may cause, in later processing, crystallization defects such as slip dislocations or the like, thereby lowering the mechanical strength of the wafer, or exerting adverse effects on the electrical properties of the wafer. Therefore, superficial layers containing such processing distortions must be thoroughly eliminated from the wafer. Usually, to eliminate such superficial layers, the wafer is subjected to etching treatment. The etching treatment can be classified into acid etching based on an acid etching solution comprising mixed acid, and alkali etching based on an alkali etching solution comprising alkali such as NaOH.
As for acid etching, its etching rate on the surface of a wafer markedly varies depending on the species of reactive molecules and concentration gradients of reaction products because the dispersive layer of the etching solution in contact with the wafer surface fluctuates in its thickness as a result of non-uniform flow of the etching solution etc. Therefore, the uniform flatness of the surface obtained as a result of lapping may be damaged during etching, and undulations having an amplitude of the order of a millimeter (mm) or indentations called peel may develop on the surface.
On the other hand, the etching rate in alkali etching is not affected by the species of reactive molecules and concentration gradients of reaction products, and thus the uniform flatness of the surface obtained as a result of lapping is retained even after etching. As long as the obtainment of a wafer having surfaces with a high flatness is concerned, etching based on alkali solution is better than the acid-based counterpart. However, when a wafer is subject to alkali etching, pits (to be called facets hereinafter) having a diameter of several to a few tens micrometers (μm) and a depth of a few micrometers (μm) may be formed on the surfaces depending on the orientation of crystal there. In alkali etching, the etching rate greatly differs according to the direction of etching with respect to the orientation of crystal, e.g., etching rates in the crystal directions <100>, <110> and <111> are widely different from each other, namely, the ratios of etching rate among the crystal directions <100>, <110> and <111> are widely different (crystal anisotropy). This crystal anisotropy is responsible for the development of facets in alkali etching. The flaw characteristic with alkali etching includes not only facets but also deep pits having a diameter of several micrometers (μm) or less, and a depth of a few to several tens micrometers (μm). If there is a dot-like local flaw or stain on the surface of a wafer, the dot-like flaw will cause reaction to proceed abnormally which may result in the development of a deep pit.
The condition of the rear surface of a wafer after etching is retained until the wafer is processed into a device. Because of this, a problem mentioned below will arise.
When the rear surface of a wafer is attached to a sucking plate for photolithography in order to fabricate a device from the wafer, undulations having an amplitude of the order of a millimeter (mm) of the rear surface formed as a result of acid-based etching are transmitted as they are to the front surface, and the undulations having an amplitude of the order of a millimeter (mm) transmitted to the front surface degrade the resolution of patterned light incident thereupon, which will in the end cause the yield of devices produced to be reduced.
On the other hand, when the rear surface of a wafer having undergone alkali-based etching is attached to a sucking plate for photolithography, the sharp, ragged edges of facets and pits formed on the rough surface of the wafer are tipped off to be scattered into air to produce numerous particles which may cause the lowered yield of device production. A wafer having undergone alkali-based etching causes the development of dust consisting of 4000-5000 particles, and a wafer etched with acid solution is responsible for the development of dust consisting of 2000 particles. In contrast, a wafer both of which surfaces have been mirror-polished evokes scarcely any particle, and does not cause the development of dust.
Thus, if both surfaces of a wafer are mirror-polished, its rear surface will have no coarse roughness that may cause the development of dust, and no undulations with an amplitude of the order of a millimeter (mm) will be evoked. This will ensure the high degree flatness of the surface. In short, such a wafer will be relieved of the problems caused by etching. However, a new problem arises in relation to the mirror-polished wafer. If both surfaces of a wafer are mirror-polished, not only its front surface but its rear surface also have a mirror surface. Because of this, a commonly used wafer detecting sensor which detects the presence of a wafer by receiving light scattered from the surface of the wafer could not detect such a mirror-polished wafer.
To solve the above problem, proposed is a method for producing a semiconductor wafer wherein etching is based on alkali solution, and a rear-surface polishing step is inserted between the etching step and a surface-polishing step, in order to eliminate part of roughness formed on the rear surface of the wafer as a result of alkali-based etching (Japanese Patent No. 2910507). According to this method, it is possible to allow a commonly used wafer detecting sensor to detect the rear surface of a wafer etched by alkali solution, to allow the front surface of the wafer to have a high degree flatness, and to raise the yield of device production.
However, the method disclosed in Japanese Patent No. 2910507 is not completely free from problems. According to this method, etching is performed using an alkali solution whose etching rate varies depending on the etching direction with respect to the orientation of crystal, and thus a wafer etched by the solution will have numerous small facets and even deep pits formed on both of its surfaces. At the succeeding mirror-polishing step, then, it is necessary to remove a sufficiently thick superficial layer from the surfaces of the wafer which may damage the flatness reached as a result of lapping.
Another problem is as follows. Assume that a wafer which has, on its surface particularly of an edge, an abnormal spot consisting of a notch, local flaw or stain as shown in FIG. 10(a) must be etched. If the wafer is etched with an acid solution which etches uniformly being unaffected by the orientation of crystal, a round etched surface is formed around the abnormal spot as shown in FIG. 10(b). On the other hand, if the same wafer is etched with an alkali solution which etches differently with respect to the orientation of crystal, abnormal reaction occurs around the abnormal spot to cause a deep pit to develop there as shown in FIG. 10(c) whose edge might be broken or tipped off to produce particles.